The present invention relates to a coil, especially for a high-voltage ignition coil circuit.
It is known that, in spark ignition internal combustion engines, the combustion of the gas mixture in the cylinder is caused by the spark which is produced between the electrodes of an ignition spark plug.
In order to produce this spark, the terminals of the spark plug are connected to the ends of the secondary (high-voltage) winding of a transformer such as an ignition coil, the primary winding of which is connected to a voltage supply via a switch such as a transistor.
When this switch is closed, an electric current flows in the primary winding. If, at a given instant the switch is then opened, a sudden overvoltage is produced in the primary winding. This generates, by induction, a voltage surge in the secondary winding. When this voltage reaches a sufficient value, a spark is produced which ignites the fuel mixture.
Since the voltage across the terminals of the secondary may reach several tens of thousands of volts, various arrangements have already been proposed in order to limit the risk of forming a spark between two turns of this winding. In general, provision is made for this winding to be wound on a holder comprising a tubular winding core and a plurality of fins perpendicular to the axis of the core. The fins between them define annular winding compartments, the bottom of each compartment being formed by the core and each fin having a passage for allowing the wire of the secondary winding to pass from one compartment to the adjacent compartments.
Various measures have already been proposed for improving this arrangement.
Thus, document EP A 0,375,502 provides isolating compartments between the winding compartments so as to increase the distance between the turns of two successive winding compartments. However, this arrangement has the drawback of increasing the axial size of the secondary winding holder.
It has also been proposed, in document EP-A-0,609,109, to give the core a shape such that the passage from one compartment to the adjacent compartment runs into the upstream compartment at a certain distance from the core and into the downstream compartment level with this core. Thus, near the passage, the windings of two adjacent compartments are offset so that it is possible to limit the voltage between two turns arranged opposite each other.
These arrangements give results which are generally satisfactory but which at the present time, however, have a drawback. This is because attempts are being made more and more to increase the voltage across the terminals of the secondary and, for this to be done, consequently to increase the number of turns in this winding. Moreover, given that attempts are being made to ensure that it still has a small diameter, it must necessarily be greater in length.
At the same time, it is not desirable, in order to limit the cost of the coil, to increase the number of fins and therefore of winding compartments. The latter are therefore being made wider and wider.
One is therefore confronted with a problem of flow of the plies of wires and of mixing of the turns.